In line with increasing technological development and demand relating to mobile devices, demand for secondary batteries has also been rapidly increased. Among the secondary batteries, lithium secondary batteries having high energy density and operating voltage and excellent preservation and lifetime characteristics have been widely used as energy sources of various electronic products as well as various mobile devices.
A lithium secondary battery uses metal oxide such as LiCoO2 as a positive electrode active material and a carbon material as a negative electrode active material, a porous polymer separator is disposed between a negative electrode and a positive electrode, and a non-aqueous electrolyte solution containing a lithium salt such as LiPF6 functions as a medium in which lithium ions move between the negative electrode and the positive electrode. The lithium secondary battery must be basically stable in an operating voltage range of the battery, must have high charge and discharge efficiencies, and must have an ability to transfer ions at a sufficiently high rate.
A lithium secondary battery having an operating voltage of 3.6 V or more is used as a power source of a portable electronic device or a plurality of the lithium secondary batteries is connected in series to be used in a high-power hybrid vehicle. Since an operating voltage of the lithium secondary battery is three times higher than that of a nickel-cadmium battery or nickel-metal hydride battery and energy density characteristics per unit weight are also excellent, the lithium secondary battery has a tendency to be used widely. In the lithium secondary battery, coating of a polymer separator may be performed in order to increase adhesion to an electrode while maintaining safety of a cell, high-temperature storage performance, and high-temperature cycle performance. However, this may cause an increase in initial resistance of the battery.